Social importance of a communication network has become greater with development of an information communication society, and a demand for communication quality and availability of the communication network is increasing. It is necessary for a communication network to perform control and management of communication in order to ensure communication quality and availability. Thus, each of communication devices constituting a communication network has information for performing control and management of communication of data passing therethrough. For example, in a communication network where communication is performed using a packet format, each of communication devices has information relating to a flow of packets. A flow indicates a group of packets having a common attribute, for example, a group of packets having transmission source and destination addresses and a protocol in common. Each of the communication devices such as relay devices constituting the communication network performs control and management of communication by, for example, controlling a communication amount and stopping communication for each flow based on flow information. Each of the communication devices constituting the communication network becomes capable of performing control and management of communication for each flow by analyzing packets passing through the own device and obtaining flow information.
In addition, as a method for enhancing availability of a communication network, for example, there is a method of using redundantly configured communication devices and lines constituting a communication network. In a communication network redundantly including an active system and a standby system, where switching to the standby system is performed in a case of an abnormality in the active system, it is preferable for the switching in the case to be performed in a short time. Thus, a communication device included as the standby system may sometimes be required to be communicable within a short time on the same condition as that of the active system before occurrence of a failure. In a redundant communication system, in order to perform switching from an active system to a standby system due to occurrence of a failure or the like while maintaining a previous communication state, it is necessary for information that each of communication devices includes to be shared between a switching source and a switching destination. Thus, a technique for sharing information between an active-system communication device and a standby-system communication device in a redundant communication system has been developed. As a technique for sharing information between an active-system communication device and a standby-system communication device in a redundant communication system, for example, a technique as in PTL 1 is disclosed.
The technique in PTL 1 relates to a connection-type communication system in which communication devices as terminating devices are redundant. The communication system in PTL 1 includes an active-system terminating device and a standby-system terminating device. The active-system terminating device transmits, upon establishment of connection with a communication device being a transmission source, an address, an initial value of a sequence number, and the like of a communication-source communication device to the standby-system terminating device. In addition, the active-system terminating device transmits an incremental value of the sequence number to the standby-system terminating device when communicating with the transmission source. By transmitting the address, the sequence number, and the like of the transmission source from the active-system terminating device to the standby-system terminating device in this way, the standby-system terminating device is able to share the same information as that of the active-system terminating device. In a case of a failure in the active system, the standby-system terminating device establishes pseudo connection based on the information taken over from the active system and starts communication without newly performing communication for establishing connection with the transmission source. PTL 1 describes that the takeover of communication in this way makes it possible to hand over communication processing to the standby system without making an own-device application module and a communication partner aware of the takeover.
In addition, PTL 2 discloses a technique relating to a communication device having a function of detecting an abnormality in a relay device in a communication network.